1. Field of the Invention
The present invention relates to an aqueous ink composition, more particularly to an aqueous ink composition which comprises as the main components a colorant and a humectant.
The present invention also relates to a method of ink-jet printing method using the above-mentioned aqueous ink composition.
2. Discussion of Background
Ink-jet printers have been widely utilized in recent years because of the advantages of low noise and low running cost, and color printers capable of producing color images on a sheet of plain paper have also been placed on the market.
However, it is extremely difficult to obtain a color ink composition which is satisfactory with respect to the requirements such as the following: image density, color reproduction, water-resistance, light-resistance, drying characteristics, blurring of images and the strike-through phenomenon, and the reliability of ink-ejection. In particular, it is necessary to control not only the wettability of plain paper by the ink composition and the penetration properties of the ink into the paper, but also the wettability of a material constituting the flow path for ink composition in the ink-jet printer (hereinafter referred to as the ink flow path) by the ink composition and the penetration properties of the ink into the above-mentioned material for path. Therefore, the formulation for the aqueous ink is of great importance because the above-mentioned physical properties are determined by the ink composition.
The addition of an alkyl alcohol having 1 to 4 carbon atoms, and a monohydric alcohol having 4 carbon atoms or less, to the formulation for the ink composition is proposed as disclosed in Japanese Laid-Open Patent Applications 6-100808 and 6-166841, respectively. Such alcohols are added to the ink composition not only to improve the image quality, but also to ensure proper wettability of the material constituting the ink flow path by the ink composition. When such proper wettability of the ink flow path by the ink composition is obtained, the ink can be readily loaded into an ink container at the initial stage (this performance will be hereinafter referred to as initial ink-loading performance), and the bubble generated in the ink container can be easily sent out therefrom (this performance will be hereinafter referred to as bubble-discharging performance). Further, the ink-refilling performance for the nozzle can be improved. However, those alcohols have odor and have the problems of safety and stability during the long-term storage.
In Japanese Laid-Open Patent Application 6-136306, an aqueous ink composition comprising a styrene-acrylic acid copolymer is disclosed. However, when the ink-jet printer with a stationary-type head is supplied with the above-mentioned ink composition, the reliability of ink-ejecting performance is unsatisfactory.
The inventors of the present invention have already proposed aqueous ink compositions, each of which comprises a specific surfactant so as to improve the drying characteristics of the obtained images and minimize the deterioration of image quality, as disclosed in Japanese Laid-Open Patent Applications 6-107990 and 6-322303. However, even though those ink compositions are employed for ink-jet printing, all of the previously mentioned requirements are not satisfied. At present there is an increasing demand for further improvement of the image density and reduction of image blurring.